In a field-emission type of cold-cathode discharge device, the geometric configuration of the cathode locally enhances the electric field such that a burst of plasma is created at the cathode when a high-voltage pulse is applied between the anode and the cathode. The plasma so created propagates towards the anode at a speed of 1-3 cm/usec, short-circuiting the anode to the cathode, typically within a microsecond.
In a triggered cold-cathode discharge device, the geometry of the cathode and anode is such that a high voltage applied thereacross is insufficient in itself to create a plasma; rather, a trigger electrode is provided which, when pulsed with a short-duration, relatively low-level trigger, creates a burst of plasma. While the instant of firing of the last-mentioned device does not require a fast rise time high-voltage pulse, as does the field-emission device, the operating characteristics of each of these devices is identical in the sense that, once a plasma is created, it rapidly short-circuits the anode to the cathode.
For high-voltage switches, the rapid short-circuiting of the anode is advantageous, because switch closure time is important. However, once the cathode plasma has been created and the switch has been closed, all control over the operation of the switch is lost, because the plasma is self-sustaining, and cannot be turned off (and the switch reopened) without removal of the high voltage between the anode and the cathode. The inability of conventional devices to selectively turn the plasma on and off, and thus electrically close and open the switch, severely limits the applications of a high-voltage switch of this nature.
Because plasma is rich in electrons, a cold-cathode discharge device can function as an X-ray source. That is to say, if electrons contained in the plasma can be drawn from the plasma and accelerated towards the anode by the potential difference between the anode and the cathode, these electrons will impact the anode with sufficient energy to produce X-rays if the anode is of high-Z material. However, this situation can exist only until the plasma short-circuits the anode, with the result that the burst of X-rays produced will have a duration typically under 1 microsecond. Thus, conventional cold-cathode discharge devices have serious limitations, whether used as high-voltage switches or as X-ray sources.
It is, therefore, an object of the present invention to provide a new and improved cold-cathode discharge device which substantially overcomes the deficiencies in the prior art devices.